Surface treatment of magnesium and magnesium base alloys



Patented Nov. 7, 1939 2,178,977 SURFACE TREATMENT or MAGNESIUM AND MAGNESIUM BASE ALLOYS Martin Tosterud,

Arnold, Pa., assignor, by mesne assignments, to Magnesium Development Corporation, a corporation of Delaware No Drawing. Application January 5, 1938, Serial No. 183,459

9 Claims.

This invention relates to a method of improving the corrosion resistance of magnesium and magnesium base alloys, and is more particularly concerned with a method for the production of protective coatings upon the surfaces of articles of magnesium and its alloys.

The term magnesium as usedinthisspecification and in the appended claims includes alloys containing more than 50 per cent of magnesium as well as magnesium itself.

While in many applications, articles made from magnesium exhibit a satisfactory stability, there are certain environments, particularly industrial or marine atmospheres, which have a corrosive effect on magnesium. Magnesium articles which have suffered corrosive attack, not only exhibit an appearance displeasing to the eye but the attack may also progress to such an extent as to cause substantial loss in mechanical properties.

For the prevention of such corrosive attack, the surface treatments of magnesium heretofore employed have not been entirely satisfactory, mainly because they afforded insufficient protection .and because they substantially altered dimen-' sions. This latter defect is most serious in the treatment of articles where the uniformity of the surface and the thickness of 'the magnesium article are critical.

The principal object of the invention, therefore, is the provision of a method whereby the corrosion resistance of magnesium articles can be substantially increased.

Another object of the invention is the provision of an improved method for treating the surfaces of' magnesium articles to produce thereon an adherent, protective coating with minimum dimensional change of the article.

The invention has as a further object the provision of av method for the production on the surface of magnesium articles of an adherent, corrosion-resistantcoating which afiords increased adherence to subsequently applied paint or other siccative coats.

I have discovered that adherent coatings of a highly protective nature and with other desirable properties can be applied to magnesium surfaces by immersion of the article to be treated in an aqueous solution containing chromate ions and a sulfate of the group consisting of zinc and cadmium, and that relatively little dimensional change of the magnesium article occurs as a result of this treatment.

The success of my method in the production of a protective coating seems to depend, as far as I have been able todetermine, on the use of the In the practice of my invention, the magnesium 5 article to be coated is immersed in an aqueous solution containing one or more of the dichromates of the alkali metals, such as sodium or potassium, or ammonium dichromate, and a sulfate of zinc or cadmium, or a mixture of 1 these sulfates, until a coating of the desired thickness is formed upon its surface. The term alkali metal dichromate as'used in this specification and in the appended claims includes ammonium dichromate as well as'the alkali metal dichromatesl The respective concentration of the dichromate and-sulfate, as well as the ratioof the amounts of dichromate to sulfate in the treating solution may vary considerably, but upon increasing the j concentration of the dichromate, more satisfactory results are usually obtained if the concentration of the sulfate is also increased. The dichromate may be used in concentrations up to I saturation at the boiling temperature, but it is preferable to use from about 50 grams per liter to about 250 grams per liter. With concentrations of about 50 grams per liter of dichromate. it is preferable to use from about 5 grams per liter to about 50 grams per liter of sulfate, and with a saturated solution of dichromate, which is about 800 grams of potassium dichromate per liter at the boiling point,,the sulfate is preferably used in amounts of about 50 gramsper liter to 500 grams perliter. Particularly good results are obtion of from about 10:1 to about 1.521 represent 40 v a good working range.

The dichromate-sulfate solutions used in the practice of my invention are effective over a wide temperature range, but .the temperature varies inversely with the period of immersion of the article'to be coatd. For example, production of satisfactory coatings atrooin temperature may require immersion periods of 24 hours or more, while an equivalent coating may be obtained with about 30 minutes treatment at the boiling temperature. For this reason I prefer to use the solution at or near its boiling point. The period of immersion at any given temperature depends on the thickness of the coating desired. When the conditions to which the coated article is to ditions corrosive conditions, the treatment should extend to 30 minutes or more to produce a thicker coating. The coating formed in this manner is hard and definitely crystalline in structure.

The; solutions having the concentrations mentioned hereinabove usually have initial pH values of between 4 and 5, which is the preferred range of acidity. However, a pH somewhat higher or lower than the preferred range may be used with satisfactory results. Upon continued use in the coating of magnesium, the solutions become less acid as a result of small amounts of magnesium dissolving therein. This decreased acidity tends to retard the reaction. The preferred pH value. may be maintained preferably by addition of an, acid, such as chromic or sulfuric.

In a specific embodiment illustrating the practice of my invention, an article of the following composition:

Aluminum 9.0-11.0%

Manganese 0.1+

Zinc' 0.3- 0.5-

Silicon Balance magnesium.

was immersed in a water solution containing 100 grams per liter of potassium dichromate and 50 grams per liter of zinc sulfate for a period of 30 minutes while maintaining the temperature at the boiling point and the pH between 4.4 and 4.8.

The article was then removed from the bath, washed and air-dried. It had a coating upon its surface which, when the article was subjected to accelerated corrosion tests, exhibited relatively high protective properties both, when used alon or as a paint base.

Itis to be understood that the operating con-- periods of treatment disclosed hereinabove are not limitations of my invention but. are to be considered merely illustrative of the practice thereof.

Iclaim:

1. A method of producing a protective coating on a magnesium surface, comprising treating the said surface with an aqueous solution containing an alkali metaL dichromate and a salt selected from the group consisting of zinc sulfate and cadmium sulfate, said solution being maintained at an acidity not greater than that initially pro-- duced by the dichromate.

2. A method of producing a corrosion-resistant coating on a magnesium surface, comprising treating the said surface with an aqueous solution containing an alkali metal dichromate and a salt selected from the group consisting of zinc sulfate and cadmium sulfate in which the pH of the solution is maintained between about 4 and about 5.

3. A method of producing a corrosion-resistant coating on a magnesium surface, comprising of temperature, concentrations, andtreating the said surface with an aqueous solution containing an alkali metal dichromate in a concentration of from about 50 grams per liter to saturation and a salt selected from the group consisting of zinc sulfate and cadmium sulfate, said solution being maintained at an acidity not greater than that initially produced by the dichromate.

4. A method of producing a corrosion-resistant coating on a magnesium surface, comprising treating the said surface with an aqueous solution containing an alkali metal dichromate in a concentfation of from about 50 grams per liter to saturation and a salt selected from the group consisting of zinc sulfate and cadmium sulfate while maintaining the pH of the solution between about 4 and about 5.

5. A method of producing a corrosion-resistant coating on a magnesium surface, comprising treating the said surface with an aqueous solution containing an alkali metal dichromate and a salt selected from the group consisting of zinc sulfate and cadmium sulfate at substantially the boiling temperature of the solution, said solution being maintained at an acidity not greater than that initially produced by the dichromate.

6. A method of producing a corrosion-resistant coating on a magnesium surface, comprising treating the said surface with an aqueous solution containing an alkali metal dichromate and a salt selected from the group consisting of zinc sulfate and cadmium sulfate in which the pH of the solution is maintained between about 4 and about-5 and the temperature at substantially the boiling point of the solution.

7. A method of producing a corrosion-resistant coating on a magnesium surface, comprising treating the said surface with an aqueous solution containing potassium dichromate and a salt selected from the group consisting of zinc sulfate and cadmium sulfate for from about minutes to about 30 minutes at the boiling temperature of the solution, said solution being maintained at an acidity not greater than that initially produced by the dichromate.

8. A method of producing a corrosion-resistant coating on a magnesium surface, comprising treating the said surface with an aqueous solution containing an alkali metal dichromate in a concentration of about 100 grams per liter and a salt selected from the group consisting of zinc sulfate and cadmium sulfate in a concentration of about gramsper liter at substantially the boiling temperature of the solution for a period of from about 10 minutes to about 30 minutes ata pH between about 4 and about 5.

9. A method of producing a protective coating on a magnesium surface, comprising treating the said surface with an aqueous solution containing an alkali metal dichromate and a salt selected from the group consisting of zinc sulfate and cadmium sulfate, the ratio of dichromate to 

